Preparing printing plates such as photopolymer flexographic plates and letterpress printing plates coated with photopolymer material (such plates in general referred to as “polymer plates”) is being more and more automated. A typical process of preparing such printing plates includes starting with a plate that has an ablatable material thereon, imaging in a digital imager to ablate the ablatable material according to imaging data, and a curing process of the exposed plate that involves exposure to light energy, e.g., ultraviolet (UV) light energy. Automation often includes inline exposure to cure the plate inline after imaging.
Polymer plates as described herein include plates for flexography made of a material such as photopolymer material that can be cured by exposure to UV, and also letterpress plates (e.g., lithographic plates) that have a photopolymer material coated thereon that is exposed by UV material to cure the coating. Polymer plates as used herein also include cylinders with photo-curable coatings thereon, sometimes called polymer sleeves or photopolymer sleeves. Note that the term “polymer” is not meant to limit the composition of the photo-curable material. Any photo-curable material that is curable by UV radiation is included.
Many polymer plates are optimized for a 365 nm curing wavelength. Depending on the speed by which the polymer plate is imaged, there has to be sufficient UV power available to do sufficient UV curing of the polymer during the inline exposure. For 4 m2/h productivity, the UV power at 365 nm has to be around 150 Watts. State of the art systems that include such so called inline exposure typically use gas filled arc lamps to generate sufficiently high levels of UV energy in the required wavelength or wavelengths. Such arc lamps consume up to several Kilowatts of electrical power to provide the required amount of exposure energy. Furthermore, as much as 98% of this energy is converted into heat or other unwanted wavelengths, and needs to be filtered out and cooled away from the polymer plate. The conversion efficiency of such arc-lamp-based inline exposure systems is usually not better than 1.5% to 2%. Furthermore, such arc lamps run at high voltages, e.g., several hundred to several thousand Volts. Furthermore, such arc lamps typically need to be cooled, e.g., water cooled in order to remove the enormous amounts of waste heat and radiation not needed for the curing process.
The combination of the need for a relatively high voltage and for water or other types of cooling is potentially hazardous.
In addition to the hazards and the waste, heating of polymer plates has a negative impact on the homogeneity of halftone screen dots, e.g., has negative impact on the appearance of a homogeneous screened area. This can further increase the amount of UV energy necessary for curing.
Furthermore, some of the wavelengths produce by such arc lamps, e.g., radiation in the UV-C range can produce artifacts such as relatively brittle screen dots that can break off the plate after as few as a few hundred or perhaps a few thousand impressions. Therefore, there is a need to filter out undesired wavelengths other than the 365 nm range of wavelengths needed to cure a plate.
Such filtering of unwanted wavelengths further reduces the curing efficiency of the 365 nm range wavelength, so that the amount of 365 nm energy necessary for curing is raised.
Arc lamps also are known to produce a light source that is relatively not very diffuse. This causes very thin and small support shoulders of screen dots on a printing plate. Often complex reflector geometries are used in an attempt to compensate for the non-diffuse nature of the light from an arc lamp.
Furthermore, arc lamps are known to have a relatively limited life time. A life of between 500 and 2000 hours is typical.